Perfluorocarbon such as octafluoropropane (hereinafter referred to as “FC-218”) or octafluorocyclobutane (hereinafter referred to as “FC-C318”) is used as an etching gas or a cleaning gas in the process of manufacturing a semiconductor device.
For producing FC-218, a method of electrolytically fluorinating 1-chloropropane (see, U.S. Pat. No. 3,709,800), a method of reacting trifluoropentachloropropane and manganese trifluoride (see, U.S. Pat. No. 2,578,721) and a method of reacting propane or propylene with hydrogen fluoride and chlorine (see, U.S. Pat. No. 5,220,083) are known. However, these methods use a compound containing chlorine as a starting material and therefore, have a problem in that chlorine-containing impurities are yielded as by-products.
With respect to the method using a starting material or the like free of chlorine, a method of electrolytically fluorinating propane (see, U.S. Pat. No. 3,840,0445) is known, however, the apparatus therefor is very complicated and the yield is low, therefore, this is not an industrially advantageous method. Also, a method of fluorinating hexafluoropropene (hereinafter referred to as “FC-1216”) to produce FC-218 is known. For example, a method of reacting FC-1216 with a fluorine gas under dilution with an inert gas and a reaction product gas (see, JP-B-62-61682) (the term “JP-B” as used herein means an “Japanese Examined Patent Publication (Kokoku)”), a method of electrolytically fluorinating FC-1216 in hydrogen fluoride (see, JP-B-62-61115), and a method of reacting a high-order metal fluoride containing at least one member selected from cobalt trifluoride, manganese trifluoride and silver difluoride (see, JP-B-62-54777) are known.
For producing FC-1216, a method of using thermal decomposition of chlorodifluoromethane (hereinafter referred to as “HCFC-22”) is known. In addition, a method of fluorinating perhalogenated C-3 chlorofluorocarbon and then dehalogenating the fluorination product to produce FC-1216 (see, U.S. Pat. No. 5,057,634) is known. These methods also use a chlorine-containing compound as a starting material, therefore, FC-1216 contains a chlorine-containing compound as impurities in many cases and FC-218 produced starting from this FC-1216 contains chlorine-containing impurities together with unreacted FC-1216 in many cases. The boiling points of chlorine-containing impurities are shown in Table 1. The impurities can be mostly separated by distillation, however, the boiling points of chloropentafluoroethane (hereinafter referred to as “CHF-115”) and FC-1216 each is approximated to the boiling point of FC-218 and therefore, separation of these impurities by distillation is very difficult.
TABLE 1BoilingStructuralPointCompound NameFormula(° C.)Octafluoropropane (FC-218)CF3CF2CF3−36.7Hexafluoropropene (FC-1216)CF3CF═CF2−31Chlorodifluoromethane (HCFC-22)CHClF2−412-Chloro-1,1,1,2-CF3CHClF−12tetrafluoroethane (HCFC-124)Chloro-1,1,2,2-CHF2CClF2−10.2tetrafluoroethane (HCFC-124a)1H-Heptafluoropropane (HFC-227ca)CHF2CF2CF3−19Chloropentafluoroethane (CFC-115)CClF2CF3−38.71,2-DichlorotetrafluoroethaneCClF2CClF2−3.8(CFC-114)Tetrafluoroetylene (FC-1114)CF2═CF2−31Octafluorocyclobutane (FC-C318)c-CF2CF2CF2CF2—−6
Similarly, octafluorocyclobutane (FC-C318) can be obtained by purifying FC-C318 yielded as a by-product at the production of tetrafluoroethylene (hereinafter referred to as “FC-1114”) or hexafluoropropene (FC-1216). For producing FC-1114 or FC-1216, a method of thermally decomposing chlorodifluoromethane is used as described, for example, in EP451793, however, by the thermal decomposition reaction, many kinds of substances are produced. Also, unreacted HCFC-22 and many chlorine-containing compounds are contained in the product. The boiling points of FC-C318 and compounds contained as impurities are shown in Table 1. Most of these reaction products and unreacted HCFC-22 can be separated by distillation. However, the boiling points of FC-1216, 2-chloro-1,1,1,2-tetrafluoroethane (hereinafter referred to as “HCFC-124”), 1-chloro 1,1,2,2-tetrafluoroethane (hereinafter referred to as “HCFC-124a”), 1H-heptafluoropropane (hereinafter referred to as “HFC-227ca”) and 1,2-dichlorotetrafluoroethane (hereinafter referred to as “CFC-114”) each is approximated to the boiling point of FC-C318. Furthermore, HCFC-124 and HCFC-124a each produces an azeotropic mixture with FC-C318. Therefore, FC-C318 reduced in the concentration of these impurities to 1 ppm by mass or less can be hardly obtained by a purification method using separation by distillation.
Other than the separation by distillation, the purification method includes extractive distillation, membrane separation and adsorption separation. However, the extractive distillation method has a problem in that the equipment costs highly and the process is complicated. The membrane separation method has a problem in that an appropriate membrane having properties necessary for separating FC-218 or FC-C318 from impurities is not known and purification to an impurity content of 1 ppm by mass or less is unavailable. The molecular sizes (calculated values at stable state structure) of FC-218 or FC-C318 and their impurity compounds are shown in Table 2. The separation by adsorption using a known adsorbent such as activated carbon, silica gel, zeolite (Molecular Sieves) or Molecular Sieving Carbon (hereinafter referred to as “MSC”), can hardly attain the purification because there is almost no difference in the molecular size and boiling point between FC-218 or FC-318 and their impurities and furthermore, there is similarity in the physical properties between FC-218 or FC-C318 and their impurities.
TABLE 2Molecular SizeCompound Name(calculated value)Octafluoropropane (FC-218)4.9 to 6.1 ÅChloropentafluoroethane (CFC-115)4.3 to 5.6 ÅHexafluoropropene (FC-1216)4.9 to 5.9 ÅChloro-1,1,1,2-tetrafluoroethane4.3 to 5.6 Å(HCFC-124)Chloro-1,1,2,2-tetrafluoroethane4.3 to 5.6 Å(HCFC-124a)1H-Heptafluoropropane (HFC-227ca)4.3 to 6.2 Å1,2-Dichlorotetrafluoroethane4.8 to 5.6 Å(CFC-114)Octafluorocyclobutane (FC-C318)5.2 to 5.8 Å
Activated carbon is effective in adsorbing and thereby removing FC-1216 which is one of impurities, but cannot separate all other impurities. In conventional purification methods, it has been particularly difficult to obtain FC-218 having a CFC-115 concentration of 1 ppm by mass or less or FC-318 having a HCFC-124 and HCFC-124a concentration of 1 ppm by mass or less.
The present invention has been made under these circumstances and the object of the present invention is to provide a purification adsorbent capable of effectively removing impurities contained in a perfluorocarbon which have been heretofore difficult to remove from a perfluorocarbon by conventional purification methods, in particular removing FC-1216, CFC-115, etc. from FC-218 and FC-1216, HCFC-124, HCFC-124a, HFC-227ca, CFC-114, etc. from FC-C318, and thereby obtaining a perfluorocarbon reduced in the contents of those impurities to 1 ppm by mass or less.
The object of the present invention includes providing an adsorbent for purification of a perfluorocarbon; a process for producing the adsorbent for purification of a perfluorocarbon; high-purity octafluorpropane or octafluorocyclobutane; processes for purifying and for producing the octafluorpropane or octafluorocyclobutane; and uses thereof.